Austrian utility uses sewer energy to heat and cool offices

AMSTETTEN, AUSTRIA – Hot water from household appliances, businesses and factories pours down the drain every day, wasting not only clean water but also heat.

In the Austrian town of Amstetten, a pilot project is underway by the local utility company to retrieve this energy from a place where normally few dare to tread: the sewer.

The project uses this energy to heat 4,000 square meters (45,000 square feet) of buildings or to cool them in the summer, allowing it to dispense entirely with gas and reduce its carbon footprint.

“Almost every week, we get a visit from a different delegation coming to look,” said Robert Simmer, the head of Stadtwerke Amstetten.

“There is a French delegation coming next week, and then a Spanish one. . . . We are starting to lack the resources; we need to beef up our personnel to handle all the tours we have to do.”

Along a 42-meter stretch of sewer where the water temperature can reach 27 degrees Celsius (80 Fahrenheit), the firm has put in place a high-tech installation.

Water running in pipes adjacent to the sewer is warmed by heat exchangers that suck the warmth out.

This warmed water is then pumped to the nearby headquarters, where an efficient heat pump fires the central heating system. A heat pump is a device found in a regular home refrigerator.

“The water that is pumped over here is clean, there is no fecal matter. The equipment inside (the sewer) is also self-cleaning,” Simmer said.

Even though extra electricity for the heat pump costs the firm €6,500 ($8,850) per year, the savings are substantial compared with what it used to spend on gas.

“We have invested €240,000,” Simmer said. “This should be recouped within around 11 years. With any other renewable energy source like solar power, it wouldn’t be any sooner.”

Amstetten’s utility company is lucky because a nearby paper factory pumps hot water into the sewer, making it warmer than normal.

In addition, the firm uses under-floor heating, which is more efficient than radiators on the walls.

But even in places without such advantages, the potential for the technology is substantial, said Florian Kretschmer from Vienna’s University of Natural Resources and Life Sciences.

“The advantage from this technology is that you have a very regional resource, and wastewater is always present,” Kretschmer said.

Similar projects exist in Germany, and Switzerland is particularly advanced, with more than 200, he said. He added that there is considerable potential.

A study done by the university and others estimates that between 3 and 5 percent of buildings in Austria could be heated using this technology, with larger buildings such as schools or office blocks particularly well-suited.

This does not sound a lot, Kretschmer admits, but combined with other clean technologies like solar and wind power, it can play a role in weaning Europe off fossil fuels.

“Of course this alone is not going to solve the world’s energy problems,” Kretschmer said. “But what we need in the future is a good mix, and energy from wastewater can play a part in this.”

One potential downside is the temperature of the sewer water might fall so far that it would affect sewage treatment plants. But this would only happen if the technology was used on a very large scale.

“Sewage treatment plants are highly dependent on temperature. If the water going into the treatment plants is cooled, then this has a negative effect on the performance of the treatment plant,” Kretschmer said.

As a result, the university is investigating the possibility of extracting heat from cleaned water downstream of treatment plants.

“In fact, that would even be positive, because it will cool the water a bit before it flows into rivers,” Kretschmer said.